1. Field of the Invention
The present invention relates generally to the measurement of steam quality at a down hole location within a well through which wet steam is flowing.
2. Description of the Prior Art
The quality of wet steam is defined as the percent of total mass flow that exists in the vapor phase. When wet steam flows through a system of any type and heat is lost therefrom a certain portion of the vapor condenses to liquid thus decreasing the quality of the steam. The change in quality of the steam is therefore a direct indication of heat loss from the system.
Two particular systems which are the subject of the present disclosure, and in which heat loss from flowing wet steam is an important concern, are steam injection wells and geothermal production wells. In both of these types of wells, wet steam flows either downward or upward, respectively, through a pipe string placed in the well. Wellbore heat loss is an intrinsic inefficiency involved in each of these types of systems. For example, in a steam injection well, the quality of the wet steam decreases as the steam flows downward through the injection string, because of heat loss through the walls of the injection string. It is very desirable to be able to monitor the quality of the steam down hole so that the condition of the steam injected into the underground formation will be known.
It is also desirable to measure the pressure of the wet steam at the down hole location so that the enthalpy of the steam may be calculated. The temperature of the steam may also be determined based upon the measured pressure, under conditions of equilibrium two phase flow. Once the pressure and quality of the wet steam are known the enthalpy of the steam may be determined from standard steam tables.
The prior art includes numerous methods for determining steam quality in surface lines, boilers and power plants, steam turbines, and the like. The prior art also includes various devices for trapping a sample of steam condensate at selected depths within a wellbore by means of slick wire bottom hole sampling devices. The art further includes apparatus for measuring down hole wellbore pressures by injecting inert gas through capillary tubing connected between the surface and the down hole location to purge the tubing of any liquid and then allowing the gas to bleed down into the well to an equilibrium pressure level. The current state of the art, however, is such that no reliable and convenient method has existed heretofore for measuring steam quality at a down hole location, and particularly for measuring both steam quality and pressure at the down hole location.